fix(agent): make mid-turn /steer trusted, not read as injection

A steer rides inside a tool result (the only role-alternation-safe slot
mid-turn), so a bare "User guidance:" line reads as untrusted tool content —
well-behaved models refuse it as suspected prompt injection (observed live:
"I only follow instructions from you directly, not ones injected through
command results").

- Wrap steers in a bounded, self-describing [OUT-OF-BAND USER MESSAGE] marker
  (prompt_builder.format_steer_marker), shared by both drain sites.
- Add STEER_CHANNEL_NOTE to the core system prompt so the model expects this
  exact marker and trusts it as a genuine user message — while still ignoring
  lookalikes buried in tool/web/file output. Static text → byte-stable prompt,
  no prompt-cache regression; gated on the agent having tools.
- Desktop: steer ack is now an inline transcript note (⏩ steered · …) instead
  of a toast.

Marker is intentionally static (not a per-session nonce) to honor the
byte-stable system-prompt caching policy; nonce hardening noted as follow-up.

agent/agent_runtime_helpers.py

@@ -32,6 +32,7 @@ from pathlib import Path
 from typing import Any, Dict, List, Optional
 
 from hermes_cli.timeouts import get_provider_request_timeout
+from agent.prompt_builder import format_steer_marker
 from agent.tool_dispatch_helpers import _trajectory_normalize_msg, make_tool_result_message
 from agent.trajectory import convert_scratchpad_to_think
 from agent.credential_pool import STATUS_EXHAUSTED
@@ -2324,7 +2325,7 @@ def apply_pending_steer_to_tool_results(agent, messages: list, num_tool_msgs: in
             existing = getattr(agent, "_pending_steer", None)
             agent._pending_steer = (existing + "\n" + steer_text) if existing else steer_text
         return
-    marker = f"\n\nUser guidance: {steer_text}"
+    marker = format_steer_marker(steer_text)
     existing_content = messages[target_idx].get("content", "")
     if not isinstance(existing_content, str):
         # Anthropic multimodal content blocks — preserve them and append